update README.md

Author: zlargon

.gitignore

@@ -1,2 +1,17 @@
-# gistpreview.github.io
-Gist HTML Preview
+# Gist HTML Preview
+
+Preview HTML file on Gist
+
+## Usage
+
+```
+GET https://gistpreview.github.io/?:gist_id/:file_name
+```
+
+## Example
+
+https://gist.github.com/zlargon/de2093842657a8b3d6d9ebb6ce0a7c59
+
+## Github API Rate Limiting
+
+Unauthenticated requests are associated with your IP address, and not the user making requests. The rate limit allows you to make up to `60` requests per hour.